1. Field of the Invention
The present invention relates generally to electrical circuit interconnection and more particularly, to a multilayer circuit structure for Radio Frequency circuits, which includes via transmission lines and planar transmission lines having the same topology enabling wide frequency band interconnection between the layers of the structure and the interconnection of two or more structures.
2. Description of the Prior Art
In RF circuit development, there often exists a need to increase the density of a circuit per volume rather than per area. For example, in airborne phase scanning antennas employing transmit/receive (T/R) modules and radiating elements, it may be necessary to pack a complicated set of circuit elements between the radiating elements and the T/R modules into a small volume of space.
A block diagram of such an airborne phased array antenna element 10 having polarization agility is shown in FIG. 1, including a plurality of elemental radiators 24, 26 and a satellite communication transmit (T) channel 12, a satellite communication receive (R) channel 14, and a radar T/R channel 16. The channels 12, 14, 16 are embodied as GaAs chips. The polarization of the T channel 12 and the R channel 14 are orthogonal to each other and operate at different frequency bands. Further, the T/R channel 16 operates at yet another frequency band, and always with vertical polarization.
Implementing the antenna element 10 of FIG. 1 requires a complicated set of circuit elements. Consequently, due to space constraints, the circuit elements comprising the antenna element 10 must often be vertically stacked on top of one another in different horizontal planes. As a result, RF signals processed by these circuit elements must be vertically fed between the stacked circuit elements that lie in different horizontal planes.
FIG. 2 shows an example of a multi-layer configuration 30 for the circuit elements shown in FIG. 1. As shown, a plurality of vertical feed throughs 32, 33 connect between various layers of the multilayer configuration 30 between the radiators 24, 26 and the chips 12, 14, 16. However, such vertical feedthroughs 32, 33 are generally limited to interconnecting only adjacent horizontal circuit layers. The feed through distance in such cases is usually quite short relative to a wavelength, and the transition can be treated as a junction mismatch problem, especially for narrow frequency band.
However, the vertical feed lines 35 interconnecting the circuit elements on non-adjacent circuit layers are relatively long, having lengths of up to a wavelength. In such cases, stand-alone transmission lines such as conventional coaxial lines must be used, which have fixed impedances and are capable of carrying a signal a pre-determined distance. However, stand-alone transmission lines take up valuable space and are impractical for high frequency operations. Furthermore, they are labor intensive and extremely costly for high volume production.
Attempts have been made at utilizing via as RF grounding lines and as DC lines in printed circuits. However, most of the attempts have been for RF transmission lines that traverse no more than a small fraction of a wavelength, and no attempt has been made for vias as parallel wire RF transmission lines that traverse more than a wavelength.
Further attempts have been made at interconnecting such circuit layers by utilizing vias as RF transmission lines. However, due to the difficulties in properly matching the transition between the via RF transmission lines and the planar transmission lines, such attempts have not been fully successful for wide frequency band transmission.
There is often radiation loss at the transition between via RF transmission lines and planar transmission lines. There has been no simple, cost effective method of suppressing such radiations.
Accordingly, it is an object of the present invention to provide an improved technique of transmitting RF signal using via transmission line through distance up to at least 0.5" at DC to Ku band with low insertion loss (insertion loss at Ku band is better than 0.5 dB).
Accordingly, it is a further object of the present invention to provide an improved method for interconnecting the circuit patterns of multilayer circuit configuration which utilized vias as RF transmission lines.
Accordingly, it is a further object of the present invention to provide an improved technique for matching the transition between via RF transmission lines and planar transmission lines to accommodate wide frequency band transmission.
Accordingly, it is a further object of the present invention to provide an improved technique of using vias as mode suppressors at the transition from via RF transmission lines and planar transmission lines to suppress radiations.
Low Temperature Co-fired Ceramics (LTCC) is a technology used as a method for producing general multilayer circuit structure, which includes providing a plurality of substrate layers. It also the object of the present invention to use this method of substrate fabrication to embody via RF transmission line structures for RF transmission and transition. The structures are constructed by punching a plurality of via holes in a predetermined configuration within each of the plurality of substrate layers. Filling the plurality of via holes of each plurality of substrate layers with a conductive material. Disposing a pattern of conductive material on the plurality of substrate layers, which forms a plurality of processed layers. Stacking the plurality of processed layers, which forms a multi-layer configuration including a plurality of via transmission lines. Firing the multilayer configuration which produces the multilayer circuit structure. Furthermore, via RF transmission lines from two or more multilayer circuit structures can be electrically interconnected by disposing a pattern of conductive material on the vias, aligning the vias and fixing via connection. The interconnection can also be accomplished with, but not limited to solder bumps, epoxies, interposers or mechanical contacts.